Charlie Jane Anders

A new TV show is like a shiny toy, mint in the box: You can't wait to start breaking it and rebuilding it. Any show that stays on the air a few years will have cast changes, but also mission creep and radical status-quo alterations. Which science fiction or fantasy show changed the most from its first episode to its last?

Please include a picture or clip for your chosen TV show, and please mention the name of the show and what changed from start to end. (Thanks to Straight Dope for the idea!)